1. Field of the Invention
The present invention relates to a multiple-rotation absolute encoder for detecting the absolute position within a single rotation of a rotating shaft of a motor and the number of rotations (multiple-rotation position) of the rotating shaft of the motor from a predetermined origin position in order to detect the rotation-angle position of an output shaft of an actuator provided with a motor and a reduction gear. The present invention more specifically relates to a multiple-rotation absolute encoder capable of being set so that overflow does not occur in the multiple-rotation value counter for counting the number of rotations, even in cases where the actuator rotates continuously in the same direction.
2. Description of the Related Art
A multiple-rotation absolute encoder, e.g., a magnetic multiple-rotation absolute encoder, is provided with a magnetic drum or a rotor having multipolar magnetization, and a pair of magnetic detection elements. The magnetic drum or the rotor having multipolar magnetization is attached in a coaxial state to a rotating shaft of a motor, with the shaft being the object of detection. The pair of the magnetic detection elements is positioned at angular positions spatially separated by 90° so that sine-wave signals differing in phase by 90° are output along with the rotation of the rotating shaft of the motor. The output of the pair of the magnetic detection elements is subjected to a calculation process, and the absolute angular position within a single rotation of the rotating shaft (the single-rotation absolute value) of the motor, as well as the number of rotations of the rotating shaft of the motor from a predetermined origin (the multiple-rotation value), are detected. Multiple-rotation absolute encoders are used for detecting the rotational position of an output shaft of an actuator provided with a motor and a wave-gear drive or other reduction gear.
An overflow-detecting circuit is mounted on the multiple-rotation absolute encoder in order to detect overflow of a multiple-rotation value counter. An overflow state occurs when the count value of the multiple-rotation value counter exceeds a previously set value, such as when, e.g., the actuator continues to rotate indefinitely in one direction. In such instances, the overflow-detecting circuit detects the overflow state, generates an alarm or the like, and reports the overflow state to an upper-level system. A multiple-rotation absolute encoder provided with an overflow-detecting circuit is disclosed in JP-A 08-50034.
The angular position of the actuator output shaft can no longer be specified after interrupting or turning on the power supply of the multiple-rotation absolute encoder when the multiple-rotation value counter overflows. A flaw is therefore presented in that the actuator cannot be used continuously.